A hydraulic circuit usually comprises one device for producing the system pressure such as a pressure pump (hydraulic pump), one pressure sensor, one pressure reservoir, one overpressure valve, one recoil valve and one or more consumers which are connected with one working valve.
According to the prior art, the hydraulic pump is controlled in the normal operation of the hydraulic circuit when the system pressure falls below a specific lower threshold value and is switched off when a specific upper threshold value has been reached or exceeded. Thereby the system is operated within a specific pressure range. Typical values for the upper threshold value are in the order of 80 bar, for the lower threshold value, on the other hand, in the order of 60 bar, for example, the system being operable with limitations between 60 bar and about 40 bar. Starting at 90 bar, the overpressure valve opens and after lowering of the pressure it closes at 65 bar, for example. Depending on the need, other values are to be taken into account for the threshold values and the limiting pressures.
When the pressure sensor signal in such a hydraulic system or hydraulic circuit is not available, or there is no longer access to the pressure system—as result, for example, of failure of the pressure sensor—the control is problematic. This results in that the system is either switched off or the hydraulic pump is permanently actuated.
This procedure has the disadvantage that due to the permanent actuation the pump is permanently loaded and eventually damaged due to the increased wear. Besides, the overpressure valve is steadily stressed whereby it is also exposed to increased wear.
The problem on which this invention is based is to outline a method for control of a pressure supply device in a hydraulic circuit, especially in a hydraulic circuit of an automatic transmission which, in case o, failure of the pressure sensor or if there is no longer direct access to the system pressure, the availability of the hydraulic circuit and a protected operation of the elements thereof is ensured.
The time and frequency of switching in of the pump and of the overpressure valve are to be specially limited in order to keep the wear as small as possible.